seconds
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Dividing the total welding and assembly work content by the takt time (187 seconds divided by 60) reveals that 3.12 operators would be needed to run welding and assembly in a continuous flow at takt. Four operators would be quite underutilized, but a redistribution of work elements will not be sufficient to eliminate the need for a fourth operator.
Our next option is to eliminate waste through process kaizen to bring the work content under the takt time ceiling. A kaizen target might be to reduce each operator’s work content to 56 seconds or less (or < 168 seconds total work content.) If that fails, use of some overtime may be
necessary. With either approach, the fourth operator and the material handler who currently moves parts between the isolated processes can be reassigned to other activities that actually create value.
To allow production to takt time and mix leveling, a pacemaker process should ideally incur little or no changeover time and change over very frequently, so the left-drive to right-drive welding-fixture changeover times will need to be reduced from the current ten minutes to a few seconds. Focused attention on improving the reliability of the second spot welder (perhaps through an improved maintenance approach) will also be needed.
Acme stamping
weld/assembly cell cycle times after process kaizen
takt time 60
seconds
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Notice that on this future-state map the four welding and assembly process boxes have been combined into one box to indicate the continuous flow. A small schematic sketch of a cell inside the process box also
indicates the cellular manufacturing idea.
Question #4. Where will Acme need to use supermarket pull systems?
Acme has decided to produce steering brackets to a finished-goods supermarket (see Question #2). Two additional supermarkets – one for stamped parts and one for coils – are necessary to complete Acme’s in-plant value stream for steering brackets.
Stamped Parts
Ideally, we might introduce a tiny stamping machine dedicated to steering brackets -- what we call a “right sized tool” -- and incorporate this mini-press into the welding and assembly continuous flow. Unfortunately, this is not possible in the immediate future because machinery of this type does not yet exist. So we need to set up a
supermarket and use withdrawals from that supermarket (pull) to control stamping’s production of left-drive and right-drive parts.
Pull system design begins with customer requirements, and stamping’s customer here is the weld/assembly cell.
The cell currently requires approximately 600 LH and 320 RH stamped parts per day. Containers for the stamped parts should be sized to allow close-to-the-fingertips placement in the cell (for example plastic bins that fit into gravity-feed racks near the operators), not primarily for the convenience of the stamping or material handling departments! Small containers allow Acme to keep both LH and RH stamped parts in the cell at all times. This further reduces LH-to-RH changeover time at the pacemaker process, where very frequent changeovers (leveling the mix) is a key lean objective.
Each container in the cell – for example a bin that holds 60 stamped parts, or about one hour of current steering bracket – will have a withdrawal kanban with it. When a cell operator begins taking parts out of another bin, its withdrawal kanban is given to the material handler so that he/she knows to go to the stamping supermarket and
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Withdrawal kanban trigger the movement of parts. Production kanban trigger the production of parts. Acme can attach a production kanban to each bin of 60 stamped parts in the supermarket. Every time the material handler removes a bin from the supermarket a kanban will be sent back to the stamping press. This instructs stamping to produce 60 parts, place them in a bin, a move it to a specified location (the “market address”) in the stamping supermarket.
Now stamping no longer receives a schedule from production control. With mapping icons the flow looks like this:
However, you may have already noticed a problem with this the pull system. With a cycle time of one second per piece and a changeover time of one hour, stamping would take one hour to set up to run only 60 seconds (60 pieces) of production. Until changeover time on the stamping press is greatly reduced, replenishing what is withdrawn from the stamping supermarket on a bin-for-bin basis is clearly not practical.
Due to changeover time, stamping needs to produce batches larger than 60 pieces between changeovers. With the initial goal of “every part every day”, stamping’s target batch size for steering brackets would be approximately 600 LH and 320 RH pieces (which will still require changeover-time reduction). Stamping will keep 1.5 days of parts in its supermarket, one-half day extra to allow for replenishment delay and some stamping problems.
So Acme will use a signal kanban to schedule stamping. In this case the kanban (often a metal triangle) for left-and right-drive parts is brought from the supermarket to the stamping press whenever the number of bins remaining in the supermarket drops to a trigger (“minimum”) points. When a triangle kanban arrives at the stamping press’
scheduling board, it initiates a changeover and production of a predetermined batch size of a specific part.
Stamping still does not receive a schedule from production control.
Drawn with icons, the flow now looks like this:
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Coils
To build as plant-level lean value stream the future state map must also show a third supermarket at the receiving dock, which holds coils of steel. Even though Acme’s steel supplier is not ready to receive kanban and produce according to them, Acme can still attach an internal withdrawal kanban to every coil and send those kanban to its own production control department whenever another coil is used. Production control can then order coils based on actual usage, instead of based on MRP’s best guess of what future usage will be. (MRP may still be used to provide a capacity-planning forecast for the coil supplier, but day-to-day orders should be based on pull.) Once production control has made the day’s order for coils, the corresponding kanban can be placed in kanban slots at the receiving dock. These indicate the day that coils should arrive. If there are kanban still left in yesterday’s receiving slot, then something is wrong at the supplier.
Currently the steel supplier is shipping coils weekly. By lining up other customers along a “milk run” delivery, it may be possible to get the necessary amount of steel on a daily basis, even if the steel supplier does nothing to reduce its minimum batch size for slitting coils. Simply moving to daily delivery eliminates 80% of the inventory at Acme, while providing smooth, steady demand for the steel supplier.